S251: P-SELECTIN CONTRIBUTES TO HEMATOPOIETIC STEM CELLS AGING BY SENSING THE NICHE
Authors: Daozheng Yang, Natalia Skinder, Arthur Flohr Svendsen, Bertien Dethmers-Ausema, Ellen Weersing, Victor Thiruthuvanathan, Maria Marianovich, Britta Will, Gerald de Haan
Journal: HemaSphere (2023)

Background: Aging-associated functional decline of hematopoietic stem cells (HSCs) is hallmarked by increased dormancy and myeloid and megakaryocytic lineage bias during steady state and regenerative hematopoiesis. While cell autonomous mechanisms as well as a changing niche have been identified as crucial drivers of HSC aging, how cell intrinsic and extrinsic pathway cross-talk facilitates stem cell aging has remained unclear. Selectins govern leukocyte recruitment during inflammation via establishing context-specific interaction patterns of different adhesion molecules on the neutrophil surface. Our recent work uncovered consistent upregulation of P-selectin (encoded by Selp) in aged HSCs; the functional contribution of this type-1 transmembrane glycoprotein to aging-associated HSC decline has remained unknown. Aims: In this study, we aimed to determine the principal roles of Selp in hematopoiesis and HSC aging, and to better understand how aberrant stem cell-niche communication may drive aging phenotypes. Methods: We determined the molecular and functional hallmarks of stem cells with high and low Selp cell surface expression (Selphi and Selplo HSCs, respectively) as well as Selp deficiency (SelpKO), utilizing RNA-seq, serial bone marrow (BM) transplantation and ex vivo stem cell division assays. To elucidate how Selp may contribute to the crosstalk between HSCs and BM niche, we characterized the distribution of Selphi and Selplo HSCs by whole-mount BM imaging, and investigated the effects of P-selectin glycoprotein ligand (Selplg) exposure on HSCs. We determined the mechanism of aging-associated Selp overexpression in an in vivo model of inflammation. Results: While barely detectable on the cell surface of HSCs isolated from young mice, Selp increases on the cell surface of aged HSCs, while other selectins are unchanged (L-selectin) or not expressed (E-selectin) in young and old HSCs. Selp expression was induced upon IL-1β administration (also elevated in the aging BM), implicating a pro-inflammatory niche as a driver of increased Selp expression on HSCs. Transcriptomic analysis of Selphi HSCs revealed enrichment of aging and low stem cell output (dormancy) gene signatures. Consistently, we found Selphi HSCs to be less prone to enter active cell cycling ex vivo and with a reduced repopulation capacity upon transplantation. Notably, Selp expression in aged HSCs declined significantly upon transplantation into young recipients further substantiating a crucial role of the aging/pro-inflammatory microenvironment. We further discovered that while Selplo HSCs seemed to have no preference for a particular stem cell niche, Selphi HSCs were significantly enriched in sinusoidal niches. Mechanistically, we found that exposure to Selplg instructs cycling of Selplo HSCs, in contrast to SelpKO and surprisingly also Selphi HSC. In line, we noted decreased cycling activity ex vivo and significant impairment of repopulation capacity of SelpKO HSCs upon transplantation compared to young wild type controls. These data implicate Selp-Selplg as a novel mechanism for cell cycle regulation in stem cells. Summary/Conclusion: In this study we identify Selp is not only a marker but also an important regulator of hematopoiesis and HSC aging. We highlight a key role for pro-inflammatory cytokine-triggered cell membrane protein composition changes in impairing HSC activation and contributing to the functional changes of aging stem cells. Keywords: Aging, Bone marrow niche, Inflammation, Hematopoietic stem cell